1. Field of the Invention
The present invention relates to a thermal barrier coating applied to the surface of a superalloy article, e.g. a gas turbine engine turbine blade, and to a method of applying the thermal barrier coating.
2. Description of Related Art
The constant demand for increased operating temperature in gas turbine engines was initially met by air cooling of the turbine blades and development of superalloys from which to manufacture the turbine blades and turbine vanes, both of which extended their service lives. Further temperature increases necessitated the development of ceramic coating materials with which to insulate the turbine blades and turbine vanes from the heat contained in the gases discharged from the combustion chambers, again the operating lives of the turbine blades and turbine vanes was extended. However, the amount of life extension was limited because the ceramic coatings suffered from inadequate adhesion to the superalloy substrate. One reason for this is the disparity of coefficients of thermal expansion between the superalloy substrate and the ceramic coating. Coating adhesion was improved by the development of various types of aluminium containing alloy bond coatings which were thermally sprayed or otherwise applied to the superalloy substrate before the application of the ceramic coating. Such bond coatings are typically of the so-called aluminide (diffusion) or "MCrAlY" types, where M signifies one or more of cobalt, iron and nickel.
Use of bond coatings has been successful in preventing extensive spallation of thermal barrier coatings during service, but localised spallation of the ceramic coating still occurs where the adhesion fails between the bond coating and the ceramic coating. This exposes the bond coating to the full heat of the combustion gases, leading to premature failure of the turbine blade or turbine vane.